Crawl space vents let outside air move through the foundation, but in humid summer air they often add moisture instead of removing it.
Foundation vents were sized on a simple assumption: outside air is drier than crawl space air. In a Gulf Coast July, that flips. Warm, heavy air enters a cooler crawl space, drops in temperature, and its moisture condenses on joists, ductwork, and subfloor sheathing.
That reversal is why building scientists treat venting as a climate question rather than a default. Vents do have a job — dilution, cross-ventilation, pressure relief — but the codes requiring them were written around dilution, not drying.
What Vents Are Supposed To Do
Vents dilute ground moisture with outside air and give air a path to cross the space rather than sit still. In a dry, cold climate that partly holds up.
The common code baseline is 1 square foot of net free vent area per 150 square feet of floor, relaxing to 1 per 1,500 square feet when a qualifying 6-mil polyethylene vapor retarder covers the entire floor. Placement matters too: openings around the perimeter on at least two opposite sides, as high above grade as practical, some summaries calling for vents within 3 feet of each corner.
Those numbers describe a moisture-dilution strategy — not drainage, not a dehumidifier, not a substitute for a ground cover.
Why Vented Crawl Spaces Still Get Damp
Ventilation moves air; it doesn’t guarantee drier air. In humid regions, summer outside air carries far more water vapor than the crawl space can absorb, and cooling it inside pushes it past its dew point.
Consequences show up in the structure: mold growth, wood decay, pest entry, shorter service life for framing and ductwork. Sealed, conditioned crawl spaces are reported to perform better across safety, health, comfort, durability, and energy use.
A vented crawl space is also an open hole in the building envelope. Every cubic foot of outside air that enters must be conditioned by the house above, and in winter that means paying to heat air at outdoor temperature.
Sealed And Conditioned: The Modern Alternative
ENERGY STAR and building-science sources favor closing the vents and making the crawl space part of the conditioned envelope. The ENERGY STAR closing sequence:
- Verify combustion-air and makeup-air needs for mechanical systems before closing anything. This comes first for a reason.
- Seal every intentional, operable, or fixed foundation vent.
- If vents sit below grade, seal from the outside with brick, block, or concrete, then insulate from the inside.
- Use rigid foam and spray foam to insulate and seal the vent opening from the inside where suitable.
- Cover the whole floor with a minimum 10-mil vapor barrier, or a cross-laminated, puncture-resistant 6-mil poly.
Two cautions: ENERGY STAR warns sealing vents can interfere with combustion-air or makeup-air needs for equipment in or near the crawl space — check first. Below-grade vents also need outside sealing plus pest-resistant barriers.
If you’re replacing failed or missing covers instead of closing everything up, this roundup of tested crawl space vent covers and options covers the hardware side.
Special Cases: Skirting And Manufactured Homes
Manufactured housing skirting has stricter venting rules. Cross-ventilation and screened openings are required, covers must resist rodents, and adjustable closures are specified for freezing climates. The 24 CFR 3285.505 skirting provisions spell out those requirements, including the screened, rodent-resistant design. Under that section, the question isn’t whether to vent — it’s whether the openings meet the standard.
| Crawl Space Approach | Summer Humidity Effect | Best Fit |
|---|---|---|
| Vented, no ground cover | Adds moisture in humid air | Dry, cold climates |
| Vented with 6-mil poly floor | Reduced ground vapor, still vent-limited | Marginal climates |
| Sealed and conditioned | Excludes humid outside air | Humid and mixed climates |
| Sealed, isolated from HVAC | Depends on dehumidification | Spaces with no ductwork |
| Manufactured home skirting | Code-driven cross-ventilation | Regulated skirting systems |
The Bottom Line
Crawl space vents do one thing: exchange inside air for outside air. Whether that helps depends on which air holds more moisture, and in most of the country’s humid months the answer is the outside air. That’s the case for sealing and conditioning: the ground vapor barrier handles moisture, the sealed perimeter stops humid air from arriving, and the vents retire.
Cornell’s CFR text on manufactured home skirting and ENERGY STAR’s closing guide are the documents to read before committing to either route, and the Oak Ridge foundation handbook crawl space section covers the moisture physics behind the guidance.
FAQs
Are crawl space vents required by code?
Vented crawl spaces are common under IRC-style rules, which set net free vent area at roughly 1 square foot per 150 square feet of floor, relaxing to 1 per 1,500 square feet with a qualifying 6-mil ground vapor retarder. Requirements vary by state and local adoption, and unvented designs are permitted under the sealed-crawl-space approach.
Should I close my vents in summer?
In humid climates, closing them usually helps. Warm outdoor air entering a cooler crawl space can cool below its dew point and condense on framing and ducts, adding moisture rather than removing it. Sealing the vents and covering the floor with a vapor barrier addresses the source instead of the symptom.
What is the minimum vapor barrier thickness?
ENERGY STAR’s closing guide calls for a minimum 10-mil vapor barrier across the entire crawl space floor, or a cross-laminated, puncture-resistant 6-mil poly. Full-floor coverage matters as much as thickness, since gaps let ground moisture straight back in.
References & Sources
- ENERGY STAR. “Guide to Closing and Conditioning Ventilated Crawlspaces” Source for the five-step closing sequence, the 10-mil and 6-mil vapor barrier specs, and the combustion-air warning.
- Oak Ridge National Laboratory. “Foundation Handbook — Crawl Space Section” Source for moisture behavior in vented crawl spaces and the performance case for sealed, conditioned assemblies.
- Cornell Law School, Legal Information Institute. “24 CFR 3285.505 — Skirting and Perimeter Enclosure” Source for manufactured housing skirting ventilation, screening, and rodent-resistant cover requirements.